Deoxidation of aluminum-killed molten steel

ABSTRACT

When aluminum-killed molten steel is deoxidized with an alloy consisting of 15-30 percent barium and/or strontium, 5-30 percent calcium, 40-60 percent silicon and/or aluminum, the balance of not more than 20 percent consisting of iron, manganese, and other impurities, the slabs, billets, and the like produced by continuous casting contain aluminum oxide as randomly distributed practically spherical inclusions. Accumulation of slag inclusions near the skin and the resulting defects in subsequent hot working are avoided, and the need for trimming or dressing the castings is reduced to a minimum.

United States Patent 1151 3,649,253

Kaess Mar. 14, 1972 1541 DEOXIDATION F ALUMINUM- 3,131,058 4/1964 036mm ..75/129 KILLED MOLTEN STEEL 3,275,433 9/1966 Hilty ..75/134 A I721 Inventor E393 K9 5. TSEPEQQELPBELMSL; 3,540,882 11/1970 01618111... ..75/129 x Germany M 3 m [73] Assignee: Suddeutsche Kalkstickstofl-Werke Aktieng q gl vI r i r ggi i PPV i Primary Examiner-L. Dewayne Rutledge Assistant Examiner.l. E. Legru I I Gm Attorney-Kelman and Berman [22] Flledi Oct. 8, 1969 211 Appl. No.: 864,881 ABSTRACT When aluminum-killed molten steel is deoxidized with an alloy consisting of 15-30 percent barium and/or strontium,

[] Formal] Apphcamm Pnomy Data 5-30 percent calcium, -60 percent silicon and/or alu- Oct. 14, 1968 Germany ..P 18 02 991.9 minum, the balance of not more than 20 Pereem consisting of iron, manganese, and other impurities, the slabs, billets, and 52 US. (:1 ..75/129, /58, 75/134 F, the like Pmduced by casting wmai" aluminum 75,134 N, 75/134 S oxide as randomly distributed practically spherical inclusions. 511 1111. c1 ..c21 7/06 C2lc 7/08 Accum'llafim Slag inclusimsfear l and esumng 58] Field ofs h 75/129 A 57 58 defects in subsequent hot workmg are avo1ded, and the need for trimming or dressing the castings is reduced to a minimum.

[56] References Cited 9 Claims, No Drawings UNITED STATES PATENTS DEOXIDATION OF ALUMINUM-KILLED MOLTEN STEEL This invention relates to the continuous casting of aluminum-killed molten carbon steel and low-alloy steel containing more than 0.0 l 5 percent aluminum, and particularly to the deoxidizing of such steel prior to continuous casting.

When blooms, billets, slabs, and like shapes are cast continuously from molten plain carbon steel and low alloy steel killed with aluminum and deoxidized by conventional methods, serious difficulties are encountered. If it is desired to arrive at a fine-grained steel, the added aluminum must be of the order of 0.030 percent to 0.080 percent, the exact amount varying with the composition of the steel in a known manner. The relatively large amounts of added aluminum are found in the cast product as inclusions of almost pure aluminum oxide which are formed during pouring and during solidification, and which are concentrated in clusters and other agglomerations.

The aluminum oxide agglomerations tend to precipitate immediately below the surface of the continuously cast metal and to cause cracks during subsequent hot rolling. Such cracks can be repaired only by an economically unbearable amount of labor spent on cleaning and dressing the hot-rolled semifinished products.

It has also been observed that sulfides tend to precipitate at the grain boundaries of continuously cast, aluminum-killed plain carbon steels and low-alloy steels. The sulfides segregated at the grain boundaries impair the mechanical strength of the ultimate product, particularly transversely to the direction of rolling.

In view of these difiiculties, it has been the almost universal practice heretofore to cast aluminum-killed, fine-grained steel in individual ingot molds. Agglomerations and clusters of aluminum oxide inclusions tend to occur less frequently in such molds and to impair the quality of the cast steel to a smaller extent. The normal conditions for casting ingots cause precipitation of the slag particles formed during pouring and solidification in a more favorable pattern. In a continuous casting process, the nonmetallic inclusion particles admitted to the liquid crater with the poured metal and nonmetallic particles formed during solidification tend to accumulate on the surface of the rapidly progressing crystallization front, and thereby to cause the difficulties indicated above.

It is a primary object of the invention to avoid the agglomeration of the unavoidable nonmetallic inclusions during continuous casting where they could cause the formation of cracks during subsequent hot working of the cast metal.

It has now been found that the shape and the distribution of the inclusions can be modified in continuously cast steel slabs, billets, and like shapes when the liquid steel is deoxidized by means of an alloy consisting of or 30 percent barium and/or strontium, 5 to 30 percent calcium, 40 to 60 percent silicon and/or aluminum, the remainder amounting to not more than percent being iron, manganese, or other impurities.

Alloys of similar composition have been used for deoxidizing steel prior to the casting of ingots in individual molds, The known alloys, when used in ingot casting, cause the slag particles to be precipitated in the ingot top where they can be removed without difficulty and at low cost. Such a mode of action is not useful in continuously cast steel shapes because of the much higher solidification rate which causes the inclusions to be accumulated just below the cast surface, as discussed above.

Quite surprisingly, it has been found that the use of the aforedefined deoxidizing alloys of this invention prevents the accumulation of an excessive amount of nonmetallic or slag particles below the cast skin, and causes the inclusions mainly to be distributed uniformly throughout the cast metal. The steel bodies continuously cast with the use of the deoxidizing alloys according to the invention are as good as steel bodies obtained from ingots conventionally cast in individual molds, and the known advantages of continuous casting over individual mold casting thereby become fully available in aluminum-killed plain carbon and low alloy steels.

Aluminum may be added to the molten steel prior to or simultaneously with the deoxidizing alloy. It is not practical to add the deoxidizing alloy simultaneously with the aluminum if steel containing much oxygen is to be treated by the method of the invention, but simultaneous addition is successful in the presence of little oxygen. If the deoxidizing alloy contains enough aluminum, it may not be necessary to add aluminum separately. In any event, an aluminum content of the deoxidizing alloy is to be considered in determining the amount of aluminum to be added separately.

It is preferred that the barium and/or strontium content of the alloy be approximately equal to the calcium content. At least some silicon should be present in the alloy if the iron content is near the indicated upper limit in order to improve the homogeneity of the alloy. If the iron content is low, silicon may be replaced entirely by aluminum. Manganese is not a necessary ingredient of the alloy, but it is a normally unavoidable contaminant, and does not unfavorably affect the result produced by the alloy when held within the limits usual for impurities in steel.

The four alloys whose nominal or approximate compositions are listed in the following table have been found to produce particularly favorable results.

The alloys of the invention are selected according to the composition of the molten steel to be treated, particularly according to the oxygen content and the desired ultimate aluminum content in the molten steel. The alkaline earth metals of the deoxidizing alloys should be present in an amount approximately double that of the desired aluminum content. The preferred ratio of alkaline earth metal to aluminum is held between 1.6 1 and 2.7 2 l. The optimum amount of deoxidizing alloy to be added to the molten steel separately from the aluminum is thus calculated from the amount of alkaline earth metal in the alloy. However, it should not be less than 0.1 percent by weight of the molten steel. The ultimate aluminum content of the steel is determined on the basis of known considerations, but must not be less than 0.015 percent if a finegrained steel is to be produced.

Steel bodies continuously cast according to the invention are free from the agglomerations of aluminum oxide inclusions which were unavoidable heretofore. Individual inclusions are found in the cast steel bodies practically to the exclusion of clusters or other agglomerations. They are approximately spherical, small, and randomly distributed in the steel. They do not consist solely of A1 0 but contain major amounts of CaO together with at least trace amounts of BaO and/or SrO.

The dispersed inclusions formed by the method of the invention are capable of dissolving substantial amounts of sulfur so that the residual sulfide precipitates at the corn boundaries are sharply reduced. This has not been achieved heretofore by means of the usual calcium bearing deoxidizing alloys such as those of the CaSi, CaAl, CaMnSi, CaSiAl types. The absorption of sulfur in the nonmetallic slag particles is uniquely related to the presence of barium and/or strontium in the deoxidizing alloy. When barium and/or strontium are present in amounts approximately equal to the amount of calcium in the alloy, the CaO content of the inclusions is high and almost equal to the aluminum oxide present, and the inclusions do not tend to accumulate in clusters or other relatively large agglomerates.

The following examples are further illustrative of the invention.

EXAMPLE 1 A built-up heat of steel corresponding to the German standard 15 Cr 3 was prepared in a basic electric-arc furnace by the two-slag process. The molten metal which weighed l metric tons received an addition of 0.06 percent aluminum metal in the furnace five minutes before tapping under conditions to produce an ultimate aluminum content of 0.035 percent. Dun'ng tapping, 0.23 percent of Alloy l were added to the ladle.

The molten steel was cast on a conventional continuous casting machine into blooms or billets having a cross section of 140 mm. square. The billets were subsequently rolled down to a square cross section of 65 mm. side length. The surfaces of the rolled bodies, which contained 0.034 percent Al, were practically free from cracks associated with slag inclusions, and only 17 percent of the rolled billet surface had to be dressed to remove minor defects.

When the metal was examined under a metallographic microscope, the nonmetallic inclusions in the rolled billets were found to consist of small, practically spherical particles uniformly distributed throughout the steel. Their composition was determined by means of an electron beam micro probe to average 54% A1 0 41% CaO, the balance being BaO, S, SiO MnO, FeO, and MgO.

When the aforedescribed procedure was repeated without adding the barium-calcium-silicon alloy of the invention to the ladle, the blooms or billets of M0 mm. square section were rolled down to 63 mm. square billets which contained 0.032 percent aluminum. The rolled surface was covered with slaginduced cracks which made it necessary to dress the entire surface before the billets could be processed further. Upon metallographic examination, it was found that coarse bands of slag were present particularly below the cast surface. The composition of the nonmetallic impurities were found to be 92% A1 0 balance SiO MnO, FeO, CaO, and MgO.

EXAMPLE 2 An alloy consisting essentially of Sr, 15% Ca, 55% Si, the balance being iron and unavoidable contaminants was added to the molten steel in a procedure otherwise identical with that described in Example l. The blooms and billets produced were as good as those described in the preceding Example. The nonmetallic inclusions had a composition in which BaO was replaced by SrO, but which was otherwise as described in Example 1.

EXAMPLE 3 When Alloy IV was used in the procedure of Example 1, the cast bodies had the desirable surface properties and distribution of nonmetallic inclusions described in the earlier Examples. The chemical composition of the slag particles was closely similar, a small amount of SrO being additionally present.

EXAMPLE 4 A lO-ton heat of steel meeting the German standard specification CrNi 6 and desired to have an ultimate aluminum content of 0.030 percent, was mixed in the ladle with 0.15 percent of the deoxidizing Alloy ll. Because of the high aluminum content of Alloy ll (30 percent as listed in the Table), it was not necessary to add additional aluminum.

The molten, deoxidized steel was cast and further processed as described in Example 1. The aluminum content of the rolled billets was 0.031 percent. The billets had a satisfactory surface and could be passed to subsequent operations without significant dressing or cleaning. Bands of alumina or slag clusters of practical significance could not be detected in the cast metal. The chemical composition of the inclusions was not materially difierent from that indicated in Example I.

What is claimed is: 1. In a method of continuously casting molten plain carbon steel or low-alloy steel, the steel being killed with aluminum sufficient to make the aluminum content of the cast steel more than 0.015 percent, the improvement which comprises adding to said molten steel prior to said casting at least 0.l percent of a deoxidizing alloy consisting of 15 percent to 30 percent of at least one alkaline earth metal selected from a first group consisting of barium and strontium, 5 percent to 30 percent calcium, 40 percent to 60 percent of at least one member of a second group consisting of silicon and aluminum, the balance of said deoxidizing alloy not exceeding 20 percent and consisting of members of a third group consisting of iron, manganese, and impurities 2. In a method as set forth in claim 1, aluminum being added to said molten metal not later than simultaneously with the adding of said deoxidizing alloy.

3. In a method as set forth in claim 1, the amount of the alkaline earth metal of said first group being approximately equal to the amount of said calcium.

4. In a method as set forth in claim 1, said deoxidizing alloy containing approximately 15 percent barium, 15 percent calcium, 55 percent silicon, the balance consisting of members of said third group.

5. In a method as set forth in claim 1, said deoxidizing alloy containing approximately 20 percent barium, 20 percent calcium, 30 percent aluminum, 15 percent silicon, the balance consisting of members of said third group.

6. In a method as set forth in claim 1, said deoxidizing alloy containing approximately 5 percent strontium, 10 percent barium, 15 percent calcium, 50 percent silicon, the balance consisting of members of said third group.

7. In a method as set forth in claim 1, said deoxidizing alloy containing approximately 10 percent strontium, 10 percent barium, 15 percent calcium, 50 percent silicon, the balance consisting of members of said third group.

8. In a method as set forth in claim 1, the combined amount of said calcium and of said alkaline earth metal of said first group being between 1.6 and 2.7 times the amount of said aluminum content of said cast steel.

9. In a method as set forth in claim 1, the amount of aluminum in said deoxidizing alloy being sufficient to make the aluminum content of the continuously cast steel not substantially less then 0.03 percent. 

2. In a method as set forth in claim 1, aluminum being added to said molten metal not later than simultaneously with the adding of said deoxidizing alloy.
 3. In a method as set forth in claim 1, the amount of the alkaline earth metal of said first group being approximately equal to the amount of said calcium.
 4. In a method as set forth in claim 1, said deoxidizing alloy containing approximately 15 percent barium, 15 percent calcium, 55 percent silicon, the balance consisting of members of said third group.
 5. In a method as set forth in claim 1, said deoxidizing alloy containing approximately 20 percent barium, 20 percent calcium, 30 percent aluminum, 15 percent silicon, the balance consisting of members of said third group.
 6. In a method as set forth in claim 1, said deoxidizing alloy containing approximately 5 percent strontium, 10 percent barium, 15 percent calcium, 50 percent silicon, the balance consisting of members of said third group.
 7. In a method as set forth in claim 1, said deoxidizing alloy containing approximately 10 percent strontium, 10 percent barium, 15 percent calcium, 50 percent silicon, the balance consisting of members of said third group.
 8. In a method as set forth in claim 1, the combined amount of said calcium and of said alkaline earth metal of said first group being between 1.6 and 2.7 times the amount of said aluminum content of said cast steel.
 9. In a method as set forth in claim 1, the amount of aluminum in said deoxidizing alloy being sufficient to make the aluminum content of the continuously cast steel not substantially less then 0.03 percent. 